I got it working in 9.9.0 beta:
http://beta.excellentonline.com/sitemapindex.xml http://beta.excellentonline.com/sitemap-1.xml
and it validates... The things I had to change:
comment out the call to "clear_cache();" in xml_sitemaps_install.php
then in "xml_sitemaps_main.php" there are two places where it uses "XmlSitemaps_Links_per_sitemap". Apparently it's trying to use a config var for how many stories to map. The problem is that the variable isn't defined. Maybe it used to be for earlier versions of Pligg.
A workaround I did was to just hardcode the number of stories:
# create_sitemap($_GET['i'],XmlSitemaps_Links_per_sitemap);
create_sitemap($_GET['i'],500);
the first line is the original that I commented out and then I just replaced it with 500 (the second line). That fixed the problem. I just started using Pligg so I don't know much about module development. I'm going to read up on it and find out how to pull variables out of the admin system and reference them in the code.
But once you make those changes, and enable the module, you should be good to go.